### Please describe the problem.
1. Building minimal variant of git-annex failed.
1. Seems, that instructions about [minimal building from source](https://git-annex.branchable.com/install/fromsource/) are out-of-date, at least because options 'xmpp' and 'dns' don't exist in git-annex.cabal.
1. Also seems, that file Utility/SRV.hs unused, may be remove it?

In git-annex.cabal 'Assistant' flag doesn't influence on list of modules, only on compile flags and dependencies. 'Assistant/\*' modules includes into base variant of building (and some 'Assistant/\*' modules turn on with WebApp flag).

### What steps will reproduce the problem?
Try to build as in [archlinux package](https://git.archlinux.org/svntogit/community.git/tree/repos/community-x86_64/PKGBUILD?h=packages/git-annex), but turn off assistant:

`runhaskell Setup configure -O --prefix=/usr --enable-executable-dynamic --disable-library-vanilla --docdir="/usr/share/doc/git-annex" -fconcurrentoutput -ftorrentparser -f-androidsplice -f-android -fproduction -fpairing -fwebapp -f-assistant -fwebdav -fs3 -f-benchmark -fdbus -fmagicmime`

`runhaskell Setup.hs build`

After that I got error:

`Utility/Mounts.hs:13:1: error:`

`    Could not find module ‘System.MountPoints’`

haskell-mountpoints package is instaled.
If turn on assistant flag, building is ok.

### Please provide any additional information below.
I don't know haskell, but try to make optional building of Assistant, see [patch](https://github.com/fftmp/git-annex-min-pkgbuild/blob/master/0002-optional-build-assistant-and-other-small-cabal-refin.patch). Seems, that it works for minimal building, but may be incomplete.
### What version of git-annex are you using? On what operating system?
6.20180807,  Archlinux


### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
Yes, use it for several years and have approx 100G repo with 10k+ files. Works fine.

> You had the webapp flag enabled and the assistant disabled. 
> That does not make sense; the assistant is part of the webapp.
> That explains why it failed to build. Disabling both webapp and assistant
> works, or only webapp and not assistant. 
> 
> Ok, fixed it to not build the webapp when the assistant flag is disabled.
> [done]] --[[Joey]] 
